Distortion Compensation for Printing

ABSTRACT

A printing machine includes a substrate and at least one printhead with at least two material deposition channels which are movable with respect to the substrate, wherein the printhead is mounted on a stage which allows rotation of the printhead around an axis perpendicular to the substrate and translation in a direction perpendicular to the print direction, and wherein during each print swath, the rotation angle and translation values of the printhead are varied in order to change the pitch and the lateral position in the direction perpendicular to the print direction of material deposited from the different deposition channels; and an algorithm which computes the required rotation angles and translation values for a given pattern of substrate distortion.

This invention relates to apparatus and methods for the manufacturing offunctional structures on dimensionally unstable substrates bydirect-write printing techniques, in particular the manufacturing ofelectronic devices on flexible, plastic substrates.

Integration of functional, electronic devices onto flexible substrateswill potentially enable new electronic products such as flexibledisplays, or low-cost intelligent labels, as well as new manufacturingapproaches, such as roll-to-roll manufacturing. However, many flexiblesubstrates, such as plastic substrates, exhibit significant dimensionalchanges when subject to mechanical stress or temperature variations bothof which occur during any manufacturing process. When a plasticsubstrate such as a thin sheet of polyethyleneterephthalate (PET) isheated it tends to shrink, and therefore the dimensions of any patternwhich has been defined on the substrate is changing as a result of theheat exposure. Given an arbitrary reference point on such a pattern onecan define an absolute distortion field, which attaches to every featureon the substrate a distortion vector equal to the difference vector ofthe real position of the feature on the substrate with respect to thereference point and its nominal position that was intended when thepattern was defined. The nominal position can, for example, bedetermined by a pattern on a photomask in the case of a lithographicpatterning process, or by a software instruction file which moves thepositioning stage of a materials deposition unit such as a printhead ina direct-write patterning system. For many low-cost, flexible substratessuch as PET or polyethylenenaphthalate (PEN) typical absolutedistortions on a substrate with dimensions of 12-14″ are on the order of50-100 μm. Such distortions cause severe problems in a manufacturingprocess which requires definition of multiple patterns on top of eachother with good registration of the features of an upper pattern to thefeatures of a previously defined pattern. In the case of a conventionallithographic patterning process with multiple mask levels it is verydifficult to compensate for this distortion, particularly if thedistortion pattern is not the same every time the process is performed,but changes when environmental conditions, process conditions, ormaterials properties vary.

Direct-write printing uses a materials deposition unit/printhead whichcan be positioned above the substrate to deliver a defined-amount offunctional material into well-defined locations, and is a usefulapproach to manufacturing of electronic devices. This technique iscapable of overcoming distortion problems because of its inherentability to compensate for the distortion by first detecting thedistortion pattern on the substrate, and then modifying the positioninginstructions for the printhead in order to compensate for the distortionand deliver the material in accurate registration with any previouslydefined pattern on the substrate. The process of detection can either beperformed by measuring the distortion pattern prior to the materialsdeposition step, or by detecting the local distortion and correcting forit on the fly.

Some distortion correction schemes have been described in theapplicant's earlier patent application US 2003/0059984. Examples ofdirect-write printing techniques to which such distortion compensationschemes are applicable are, but not limited to, inkjet printing,direct-write laser patterning/ablation, or dispensing. Distortioncompensation schemes can also be implemented in certain adaptivelithography techniques such as, for example, described Zemel, et al.,Proceedings of Printed Circuit Expo IPC, Long Beach, Calif., Mar. 26-28,2002.

In order to achieve high-throughput manufacturing the materialsdeposition unit needs to have many parallel channels. In the case of aninkjet printhead each heads comprises a large number (several 100's) ofnozzles, which can be operated independently from each other, and ahigh-volume manufacturing inkjet printer will need to have severalindividual printheads mounted together. The nozzles on a printhead aretypically arranged in a linear array with a well defined nozzle pitch don the order 250 μm. For example, in order to achieve a minimum TACT,the printer should ideally be able to define an arbitrary pattern on thesubstrate in a single pass. In the direction perpendicular to the printdirection several printheads may be mounted in order to cover the fullwidth of the substrate.

In such a multiple head assembly complex distortion compensationalgorithms can be employed in order to ensure that an arbitrarysubstrate distortion can be compensated for. One scheme is illustratedin FIG. 1. N printheads are mounted behind each other each of which isshifted by a defined distance d/N perpendicular to the print direction.In the ideal case in which the substrate is undistorted and the pitch ofthe pattern to be printed is equal to the nozzle pitch (if the latter isnot the case the head can be raked by a certain angle to achieve thedesired pattern pitch), the pattern can be defined entirely by firingthe nozzles of one print head only. If the substrate is distorted andthe printed pattern needs to compensate for this distortion each portionof the pattern can be printed with that nozzle on a particular printheadwhich comes closest to the required position (FIG. 1), when moving theprint head assembly along the print direction. If N is chosen such thatd/N is smaller than the required registration accuracy an arbitrarydistortion pattern can be compensated for in this way.

A significant drawback of this scheme is that it requires a large numberof printheads, and more importantly, a large number of nozzles will notbe firing regularly, but will remain idle for significant periods oftime. In the case of inkjet printing, drying of the ink on the nozzleplate is a serious problem, which can cause degradation of thedirectionality of the droplets emitted from the nozzle, or in the worstcase, nozzle blockage. In order to avoid such problems it is desirablethat all nozzles are in use regularly, and that no nozzle remains idlefor prolonged periods of time. In the distortion compensation scheme ofFIG. 1 this requirement is difficult to meet.

It is known, from US 2002/0105688, to provide a printer with a pluralityof heads, each rotatable and slidable, with control of nozzle row angleand nozzle row spacing, for fabricating an electroluminescent device.Reference is particularly made to FIG. 4 of this application and thetext at paragraphs 143-147. A technique for distortion compensation in ahand-held ink jet printer is described in U.S. Pat. No. 5,593,236 andU.S. Pat. No. 5,988,900 in which, broadly speaking, rollers are used todetermine a position of the print head with respect to the medium uponwhich the head is to print. Compensation for distortion caused bymechanical image stretch, in particular uni-directional stretch causedby transport of paper through an ink jet printer, is described in US2004/0155948.

According to a first aspect of the present invention there is thereforeprovided a controller for controlling direct-write apparatus, theapparatus comprising at least one direct-write head with a plurality ofdirect-write channels for writing onto a substrate, at least one of saiddirect-write head and said substrate being moveable relative to theother in a writing direction, said head being further positionablerelative to said substrate by at least rotation about an axissubstantially perpendicular to said substrate at a writing position ofsaid head; the controller comprising: a distortion data input to receivesubstrate distortion information characterising distortion of saidsubstrate to be written; a distortion compensation module to determine,responsive to said distortion information, head adjustment data forcontrolling said head rotation to compensate for said substratedistortion; and an output to output data for controlling said printingapparatus responsive to said head adjustment data.

Preferably said direct-write apparatus comprises droplet deposition-typeprinting apparatus, said direct-write head with a plurality of channelscomprises a droplet deposition-type print head with a plurality ofnozzles, and said writing comprises printing.

Preferably the printing apparatus also enables control of print headtranslation in a lateral direction substantially perpendicular to theprinting direction, and preferably the distortion compensation module isconfigured to determine print head adjustment data comprising bothrotation data and translation data for the print head. In embodimentsthe controller is configured to control the printing apparatus insubstantially real time—that is preferably the controller is configuredto control the print head rotation, and preferably translation, duringprinting. The printing apparatus may comprise a printer in which thesubstrate is mounted on a print head and in which either or both of theprint head and print bed are moved to effect the printing. Alternativelythe printing apparatus may comprise a roll-to-roll type printingapparatus.

The substrate distortion information may comprise pre-determinedinformation, for example from a data file but preferably the substratedistortion information is determined for each substrate to be printed,and optionally on a plurality of occasions at a plurality of differentprinting stages. The distortion information may comprise pre-processedinformation such as a two-dimensional map of the substrate distortionas, say, a distorted grid or as a series of strain vectors, or it maycomprise other information such as a captured image of a previouslyprinted pattern. Such pre-processed distortion information may have beenacquired by mapping the substrate to be printed on a separate distortionmeasurement system, or by measuring the substrate distortion on-the-flyduring printing. Alternatively, the pre-processed distortion informationmay have been acquired on a different substrate with a distortionpattern which is similar to that of the substrate to be printed. Thepreviously printed pattern may comprise an earlier layer of printing ora pattern of fiducial marks imposed upon the substrate. In either case adistortion map may be determined, for example, by comparing a capturedimage with reference (image) data.

The controller may output distortion compensation data for use by aprinter driver in applying distortion compensation to data defining apattern to be printed; alternatively the controller may receive datadefining a printed pattern and output data for driving the printer inaccordance with this pattern after having applied compensation forsubstrate distortion.

In some preferred arrangements a printer with multiple print heads isemployed (so that no raster scan in a lateral perpendicular to theprinting direction is needed), and in this case the controller may beconfigured to control the rotation and translation of the print headstogether to reduce the effects of substrate distortion. In particularone or more of the print heads may be translated laterally to effectboth compensation for substrate distortion and also compensation for anypixel pitch spacing changes resulting from print head rotation.

Preferably the controller is configured to compensate at least partiallyfor pitch variations in a lateral direction (substantially perpendicularto the printing direction). Such a pitch variation may be determined ordefined as a variation in pixel, line, or (droplet) dot pitch or interms of the pitch of some other regular and/or periodic pattern. Thelateral pitch distortion will generally vary in both the lateraldirection and the longitudinal or printing direction and preferably,therefore, the controller is configured to compensate for pitchvariations (that is distortions) of at least lateral pitch in both thesedirections. Preferably, therefore, the controller is configured tooutput data for controlling rotation of the print head (or multipleprint heads) responsive to distortion information characterising lateralpitch distortion at a position of the print head on the substrate; thismay comprise, for example, a centre or average position of the printhead. Thus the (lateral) pitch distortion can be compensated with agranularity determined by the (lateral) dimension of a print head,substantially the same compensation being applied over this (lateral)dimension. Preferably the local (lateral) pitch distortion compensationis determined for each print head. An average pitch distortioncompensation over a length in the printing direction may be applied butpreferably the pitch distortion compensation is repeatedly orsubstantially continuously adjusted as printing progresses, that is asthe print head or heads move along the printing direction.

The displacement of a print head or heads may be altered to compensatefor changes in the head rotation and is preferably also adjusted (formultiple heads) to give the desired pitch at the one or more boundariesbetween print heads—in other words to give correct line spacing at headboundaries. Optionally one or more incremental head displacements may beapplied as printing progresses, that is with motion in the printingdirection, to compensate for a global (average) deviation of desiredprinting line directions from the actual printing direction as printingprogresses.

In some preferred embodiments the controller is also configured tocompensate for distortions of longitudinal pitch that is distortions ofpitch in the printing direction. This may be achieved by outputting datafor adjusting or controlling droplet deposition timing. This data maycomprise timing signals or, in some preferred embodiments timing adjustsignals such as a frequency and/or phase adjust signal or a signalspecifying a division ratio for a high frequency signal which is to bedivided down to derive print head drive waveforms. Separate timing ortiming adjust data may be provided for each print head nozzle in such away as to facilitate the derivation of nozzle drive signals withappropriate timing from one or a few (for example one per head) variablefrequency signal generators.

In some preferred embodiments the controller includes data memory forstoring the distortion data information and instruction memory storingprocessor implementable instructions. A data processor is coupled to thedata memory to the instruction memory, to the distortion data input andto the output to load and process the instructions to implement thedistortion compensation module to process the distortion information todetermine the print head adjustment data and to determine and output thedata controlling the printing apparatus.

In another aspect the invention provides a method of compensating forpitch distortion of a substrate onto which material is to be depositedby a printer having at least one print head with a plurality of materialdeposition channels, said pitch distortion comprising distortion of thepitch of a pattern with respect to which said deposited material is tobe aligned, the method comprising: adjusting a rake angle of said printhead during printing to compensate for said pitch distortion, said rakeangle comprising an angle of said print head to a direction of saidprinting.

In aspects and embodiments of the invention the printing apparatus maybe replaced by other direct-write apparatus and the print head and itsnozzles by a direct-write head with a plurality of direct writechannels, for example patterning replacing writing/deposition.

In a further aspect the invention provides a method of compensating forpitch distortion of a substrate which is to be written by a direct-writeapparatus having at least one direct write head with a plurality ofdirect-write channels, said pitch distortion comprising distortion ofthe pitch of a pattern with respect to which said writing is to bealigned, writing with said direct-write channels being performed inaccordance with a write drive waveform for each channel, the methodcomprising: moving at least one of said substrate and said head relativeto the other in a writing direction during writing; and altering one orboth of a frequency and a phase of each said drive waveform during saidwriting in response to distortion data providing information on saidpitch distortion over a two-dimensional region of said substrate tocompensate for pitch distortion in said writing direction.

The above described methods and controller may be implemented usingcomputer program code on a data carrier such as a disk, programmedmemory such as read-only memory (firmware) or on a data carrier such asan optical or electrical signal carrier. Embodiments of the abovedescribed controller and method may be implemented on any conventionalgeneral purpose computer system, or on a DSP (Digital Signal Processor)or on an ASIC (Applications Specific Integrated Circuit) or FPGA (FieldProgrammable Gate Array). Thus code (and data) to implement embodimentsof the invention may comprise code in a conventional programminglanguage such as C, or code at a lower level, or code for setting up orcontrolling an ASIC or FPGA, or code for a hardware description languagesuch as Verilog (Trade mark). As the skilled person will appreciate suchcode and/or data may be distributed between a plurality of coupledcomponents in communication with one another, for example on a network.

Embodiments of the invention will now be described by way of exampleonly with respect to the following figures in which:

FIG. 1 illustrates a conventional distortion compensation scheme;

FIG. 2 illustrates an embodiment of a distortion compensation schemeaccording to the present invention;

FIG. 3 illustrates different distorted patterns to be printed: (A) apattern of continuous lines with variable line pitch across thesubstrate; (B) a pattern of islands with variable pitch in both an x andy direction;

FIG. 4 (A) shows droplet positions for k=0 and k=K laid on top of eachother showing change of y-pitch from 176 to 179 μm; and (B) showsdroplet positions for subsequent trigger pulses for m=100-97 nozzle,showing that distance between subsequent droplets varies by less than 1μm in spite of the continuous rotation of the rake angle of the head;

FIG. 5 shows a schematic diagram of an example of a calibration patternfor a method to measure distortions on a flexible substrate;

FIG. 6 shows plots of measured distortion on a 60×80, 50 dpi activematrix substrate on PET;

FIG. 7 shows plots of the distortion of FIG. 6 after correcting each rowof pixels for an average distortion in the y-direction;

FIG. 8 shows example signal waveforms for triggering of neighbouringnozzles in order to compensate for distortion of the pitch along theprint direction; and

FIG. 9 shows an example of a controller configured to implement methodsaccording to embodiments of aspects of the invention.

Broadly, we will describe a printing machine comprising a substrate, andat least one printhead with at least two material deposition channelswhich are movable with respect to the substrate, wherein the printheadis mounted on a stage which allows rotation of the printhead around anaxis perpendicular to the substrate and translation in a directionperpendicular to the print direction, and wherein during each printswath, the rotation angle and translation values of the printhead arevaried in order to change the pitch and the lateral position in thedirection perpendicular to the print direction of material depositedfrom the different deposition channels; and an algorithm which computesthe required rotation angles and translation values for a given patternof substrate distortion.

More particularly we will describe a distortion compensation procedurefor a multiple-nozzle, multiple-head direct-write printer which enablescompensation of a range of arbitrary distortions with a minimum numberof printheads, and regular use of all nozzles of the assembly. Themethod is based on mounting each printhead on a rotation-translationstage and adjusting both the rake angle of each head, and the lateralposition of each head in the direction perpendicular to the printdirection continuously during the printing step in order to compensatefor the distortion of a pattern on the substrate with respect to whichthe printed material deposits need to be aligned accurately.

According to an embodiment of one aspect of the invention there isprovided a method in which a distortion of the pitch in the y-directionΔy(x,y) of a periodic pattern is compensated for x being along theprinting direction. The printer unit is designed such that severalprintheads are arranged in an essentially linear fashion to cover acertain swath width in the y-direction perpendicular to the printdirection. Preferably, the swath width is equal to or larger than thewidth of the substrate such that the pattern can be printed in a singlepass. Each printhead of the N printheads is mounted on a mechanicalpositioning stage which allows the n^(th) printhead (a) to be rotatedindependently, preferably by an arbitrary rake angle α_(n) (at leastover a range of angles), and (b) to be translated independentlyperpendicular to the print direction by a distance s_(n) (FIG. 2). Inaddition, in embodiments the positioning stage can allow other motionsof the printhead such as adjustment of the vertical print distancebetween the nozzle plate and the substrate plan or the lateral positionof the printhead along the print direction. The rake angle α_(n) of eachhead and the y-position of each head is preferably continuously adjustedin substantially real time during the printing to match the localy-pitch perpendicular to the print direction.

Consider a pattern of quasiparallel lines to be printed (FIG. 3A). Inthe absence of distortion the rake angle of all heads would be the same,and assuming that the lines are parallel to the print direction would beset such that d·cos α=Δy=constant. The nozzles are firing continuouslyand the print speed and firing frequency of the nozzles are adjusted soas to obtain a continuous line deposit of material on the substrate. Insome case the lines may be printed in multiple passes by filling in gapsleft in previous passes. This may be necessary if the wetting conditionson the substrate are such, that continuous line deposits cannot befabricated in a single pass. However, if the underlying pattern on thesubstrate is distorted the local pitch Δy(x,y) between neighbouringlines should vary smoothly over the substrate in order to provideaccurate registration with the distorted substrate pattern. Such asituation arises for example when printing the conducting gate or datainterconnects on an active matrix display, where thin film transistors(TFT) are located on each pixel of the display. All the transistor gatesin one row are connected by a straight high-conductivity gateinterconnect line. The position of each of the gate electrodes needs tooverlap with the channel of the TFT defined by the pattern ofsource-drain electrodes that was predefined on the substrate. If thesubstrate distorts in between the source-drain and gate patterning step,the pitch between two gate interconnecting lines should be adjusted bothin the direction perpendicular to the print direction, and also alongthe print direction. In many practical situations the variation of thelocal pitch over the length scale of one single printhead is negligible,i.e. can be neglected compared to the required tolerance for gatepositioning (see distortion data below), but becomes significant overlarger distances.

At the beginning of each swath each printhead is set to a different rakeangle taking into account variations of the line pitch over the fullswath width. Preferably, the print direction is chosen to be along thedirection of the lines. The rake angles α_(n) are initially adjustedsuch that each printhead matches the local line pitch at the edge of thesubstrate (x=0):

d·cos α_(n)(x=0)=Δy(0,y _(n))  (1)

where y_(n) is the y-position of the center of the n-^(th) printhead,and Δy(0,y_(n)) is the y-pitch of the pattern to be printed averagedover the region which the n^(th) printhead is covering at the beginningof the print swath (x=0).

During each print swath the rake angles α_(n) and translation valuesy_(n) are continuously adjusted as the printhead assembly is movingalong the print direction to adjust for the variations of the line pitchalong the print direction:

d·cos α_(n)(x _(n))=Δy(x _(n) ,y _(n))  (2)

where Δy(x_(n),y_(n)) is the y-pitch of the pattern to be printedaveraged over the region which the n^(th) printhead is covering at aposition x_(n) of the print swath.

Simultaneously, the translation stage(s) in the y-direction are used toshift each printhead by an amount s_(n) in the y-direction aiming toensure that the array of lines printed by the assembly of printheads isin the correct position with respect to the underlying pattern on thesubstrate perpendicular to the print direction. This can be achieved bya range of techniques.

For example, for each set of rake angles the values of s_(n) can bechosen such that the distance between the last, M^(th) nozzle of then−1^(th) head to the first, −M^(th) nozzle of the n^(th) head projectedon the y-direction (FIG. 2B) matches the local pitch Δy(x_(n),(y_(n+1)+y_(n))/2) at the edge between the two printheads duringprinting. Other techniques can also be used as well. Note that anyadjustment in rake angles during the swath should be accompanied with anadjustment of s_(n).

By adjusting the values of s_(n) continuously (with motion in the printdirection) it is also possible to adjust for deviations of the directionof the lines from the print direction. This might arise if the array oflines makes an angle β with the print direction, either because ofinitial misalignment of the substrate or because of a distortion of thesubstrate which requires the direction of the lines to change across thesubstrate without necessarily requiring an adjustment of line pitch. Inthis case, all printheads are continuously adjusted by the same amountsuch that ds_(n)/dt=v·tan β, where t is the time; in embodiments thiscan be a result of using the previously described method. If β isconstant across the substrate s_(n)=v·tan β·t+s_(n)(t=0) independent ofn. Note that this overall translation of the values of s_(n) can besuperimposed onto any relative adjustments of the values of s_(n) tocompensate for any variation of Δy(x_(n),y_(n)) over the width of theprint swath. If β varies in a complex manner across the substrates_(n)(t) follows a more complex solution of the differential equation,and can also depend explicitly on n.

In this way it is possible to print an array of quasi-parallel lineswith arbitrary variation of line pitch along the print direction, andwith a variation of line pitch perpendicular to the print direction aslong as the latter is sufficiently small on the length scale of one ofthe printheads, i.e. that the variation of Δy on the length scale of oneprinthead is smaller than the registration accuracy which needs to beachieved. This is in fact satisfied for many practical flexiblesubstrates, including low-temperature plastic substrates, such asheat-stabilized PET and PEN as long as the maximum process temperaturedoes not exceed values on the order of 150-200° C. Compared to thedistortion compensation scheme of the prior art our method has theadvantage that no additional printheads are required to implement thedistortion compensation, and essentially all nozzles are firingcontinuously.

One desirable feature for this technique is to ensure that even for thetwo outermost nozzles of each head which experience the largestrotational motion as the rake angles are adjusted continuously thedistance between two subsequently fired droplets along the same linedoes not significantly change. Otherwise, there can be a risk of breaksin the line arising if the distance between neighbouring dropletsexceeds a critical value.

FIG. 4 shows simulations of the droplet positions for a particularprinthead for different phases k of the trigger signal. For thissimulation we made the following typical assumptions:

-   -   Substrate size: 1 m² substrate    -   Print speed v=1 m/s    -   Nozzle pitch d=250 μm    -   Firing frequency f=32 kHz: This implies that for the 0^(th)        nozzle at the center of the printhead the x-position on the        substrate of the droplets fired by the k^(th) trigger pulse of        the nozzle is equal to k·L_(x)=k·v/f=k·31.25 μm.    -   Assume local substrate y-pitch changes by 1% from one end of the        substrate to the other end, for example from 176.8 μm near one        edge x=0 (α₀=45°) to 179.3 μm near the other edge x=1 m        (α_(K)=44.2°).    -   For simplicity pitch for firing the k_(th) droplet is assumed to        change linearly, i.e, α_(k)=α₀+(α_(K)−α₀)·m/N, where K=32000

Under these assumption we obtain for the position (in μm) of the k^(th)droplet from the m-th nozzle:

(x _(k) ,y _(k))m=(m·250·sin(α_(k))+k·L _(x) ,m·250·cos(α_(k)))  (3)

FIG. 4(A) shows the droplet positions for k=0 and k=M laid on top ofeach other showing change of y-pitch from 176 to 179 μm. FIG. 4(B) showsthe droplet positions for the m=100-97 nozzles for subsequently printeddroplets, showing that the distance between subsequent droplets variesby less than 1 μm as the rake angle is smoothly varied during the pass.

An important component of any distortion compensation scheme is theefficient measurement and detection of the distortion either during apre-patterning distortion mapping step, or in-flight during thepatterning. It is not necessary to measure the distortion of any featureof the pattern, but only on the length scale over which the uncertaintyin position due to distortion becomes larger than the required relativealignment and registration accuracy. In the following we describe aparticular embodiment for the efficient measurement of the distortionpattern and the calculation of the print positions which are required tocompensate for the distortion.

The substrate is a regular arrayed pattern for an active area displayback plane formed on the plastic substrate. It has 60×80 pixels with aresolution of 50 dpi, i.e. the pixel pitch is 500 μm. This nominalpattern is assumed to be dimensionally precise as it is defined by apattern on a photomask which is transferred onto the substrate byphotolithography.

Three alignment marks/rotation & calibration points at the corners ofthe pattern are used to calculate the sample rotation (with respect tothe machine/stages) and simple linear calibration factors in the X and Yaxes, see FIG. 5. The calibration factors are calculated by inputtingthe known dimensions for these axes, taken from the photolithographymask dimensions and comparing these to the actual positions for thethree corner points. The print direction is chosen to be the directiondefined by the two alignment marks along the x-axis, or alternativelythe y-direction.

Once the panel (substrate) is rotated and calibrated we move thepositioning stages to designated locations (taken from the maskdrawings) to view the location of the pattern. In this case we measureevery 10^(th) feature along a particular direction. On a substrate withhigher (smaller) distortions, a larger (smaller) number of features areused. Using high magnification optics and graticule we measure theoffset between expected pattern positions against actual positions, thuscreating a map of the distortion of the panel. Automated patternrecognition techniques may be employed. Note that the data is not theabsolute distortion of the substrate since it includes the initialcalibration constants for the X and Y axes.

Samples containing arrayed source-drain patterns with pitches of 500microns of 80-columns by 60-rows have been measured (hence knownX=39,500 μm and known Y=29,500 μm).

FIG. 6A shows a mesh plot of the X and Y positions of the patterns withthe distortions magnified by a factor of 300. FIG. 6B shows this data inthe form of a quiver plot. The arrows represent the magnitude anddirection of the distortion of a particular pixel with respect to itsnominal position. The length of each arrow is multiplied by a factor of300. FIG. 6C is a histogram representation of the frequency of theabsolute distortion. The plastic PET substrate exhibits maximumdistortion of about 7 μm on a length scale of about 2″. Examining FIG.6B closely we note that the length of the arrows increase towards thetop right corner of the patterned area, implying that there must be alinear distortion component present in the measurement.

The measurement data shown here for a PET substrate shows clearly thatthe distortion of the substrate on the length scale of the printhead(2″) is comparable to the droplet placement accuracy which can beachieved with a technique such as inkjet printing.

This means that with typical process conditions for PET it is notnecessary to compensate for variations of Δy(x_(n),y_(n)) over the widthof the printhead, at least as long as the substrate process temperatureis kept below 150° C., and the substrate is heat-stabilized with thehelp of a pre-processing temperature anneal during which most of thesubstrate shrinkage can be made to occur before beginning the processingof the patterned layers.

A simple example algorithm to determine the optimum y-position for theprinted gate lines is to average the y-position of all measured pixelsin each row along the print (x) direction, or over some distance alongthe x-direction over which the head rake angle is effectively keptconstant. We illustrate this method here first for single nozzleprinting. The gate lines are printed at a y-position that corresponds tothe average y-position of pixels in a particular row. In this way themagnitude of the distortion can be reduced significantly. FIG. 7A showsa mesh plot for the distortion data adjusted for the averagey-distortion in each row, and FIG. 7B shows the respective histogram.Note that the overall distortion values are now reduced to below 4 μm.This technique may be employed after mapping a substrate to createcorrected electronic data files, to improve the resolution of thedirect-write process on plastic substrates.

The method can be extended to multiple nozzle printing. For multiplenozzle printing the set of average y-positions of the pixels in each rowaveraged over a certain distance along the print direction are used tocalculate the average pitch, and rake angle of the printhead as well asthe optimum y-position of the centre of the printhead as describedabove.

We now describe a method by which distortions of the x-pitch both alongthe print direction as well as perpendicular to the print direction canalso be compensated for. This is helpful for the somewhat morecomplicated case where a two-dimensional periodic pattern needs to beprinted that has a well defined pitch Δy perpendicular to the printdirection, but also a pitch Δx along the print direction (FIG. 3(B)).Such a situation arises, for example, for the definition of thesemiconducting active layer island of the TFT on an active matrixdisplay. This pattern nominally comprises a rectangular or quadraticgrid where semiconducting material needs to be deposited in the form ofan active layer island on each node of the grid. In this example eachisland should be isolated from and not connected with neighbouringactive layer islands/nodes. In this case the print direction can bechosen along one of the edges of the grid, defined to be thex-direction. As above the technique of adjusting rake angles andtranslation vectors for each printhead independently can be used toadjust to variations of the local y-pitch Δy(x_(n),y_(n)) in both theprint direction as well as perpendicular to the print direction.

To compensate for the variations of the pitch Δx(x_(n),y_(n)) across thesubstrate several methods can be employed. One is to use a printheadeach nozzle of which can receive an independent, arbitrary waveformtrigger signal to fire droplets at any desired position. In this methodrake angles and translation vectors for each printhead are adjusted tomatch the local y-pitch Δy(x_(n),y_(n)), and independent triggerwaveforms are applied to each nozzle on each head to ensure that eachdroplet is fired at the right time to land in the correct location ofthe distorted pattern along the print direction. This method is capablefor correcting for an arbitrary distortion Δx(x_(n),y_(n)). However, onedisadvantage of this method that it uses relatively complex driverelectronics, which can apply independent trigger signals to each nozzle.Also it puts some constraints on the design of the printhead itselfsince in some designs where neighbouring nozzles share part of theactuator mechanism there is crosstalk between neighbouring nozzles, suchthat they cannot be fired at an arbitrary time difference with respectto each other.

A simplified alternative scheme is to use the same firing frequency ffor all nozzles on a particular head, but to make the phase of thetrigger signal to be independently adjustable for each nozzle duringeach swath (see FIG. 8A). The frequency f is adjusted such thatΔx(x_(n),y_(n))=v/f and needs to be variable during each swath tocompensate for variations of Δx(x_(n),y_(n)) along x. For example, bychanging the firing frequency from 32 kHz to 31 kHz the local distanceΔx between two neighbouring/subsequently fired droplets can be changedby 1 micron for a stage speed of 1 m/s (from 31.2 to 32.2 microns). Eachprinthead can have a different frequency to compensate for variations ofΔx(x,y) along y. The phase of the trigger signal applied to each nozzleis preferably adjusted independently. This is to ensure that for a givenrake angle of the head set to match the local Δy(x_(n),y_(n)) pitch eachnozzle is fired at the correct time for the droplet to land in thecorrect position in the unit cell/pixel. Note that simultaneously fireddroplets generally only land in the correct position for a number ofdiscrete values of the rake angle (since this affects the x-spacingpixel/drop pitch). Since in the distortion scheme proposed here, therake angle is varied to match the local y-pitch of the pattern, it mayoften not be possible to fire the droplets simultaneously if one wantsto ensure that they land in the correct position of the unit cell/pixel.

Compared to using complete waveforms for each nozzle this methodrequires less complex driver electronics. In embodiments it uses aprogrammable phase shift circuit at the input to each nozzle, as well asa tunable frequency generator for the trigger signal of each printhead.The phase signal applied to each nozzle is determined on the basis ofthe distortion map which has been acquired either prior to the printstep, or is being acquired in flight during the printing. The method ofvariable phase shift preferably allows firing all nozzles at the maximumfrequency compatible with the mechanical design of the printhead, andtherefore enables a high throughput.

A further alternative method for compensation of distortion of thex-pitch is to use a base trigger signal at a high frequency and a fixedphase common to all nozzles of a particular head, but fire each nozzleonly at each p-th cycle of the base trigger signal (see FIG. 8B), suchthat Δx(x_(n),y_(n))=p·v/f. Each nozzle can be fired at a differentcycle of the base trigger signal to aim to ensure the correct positionof each droplet in the unit cell/pixel. In this method it is evenpossible to compensate for variations of Δx(x_(n),y_(n)) on the lengthscale of the printhead by choosing different values division ratio of pfor the different nozzles of a printhead. To compensate for variation ofΔx(x_(n),y_(n)) along the print direction either p or f can be varied.To compensate for longer scale variation of Δx(x_(n),y_(n))perpendicular to the print direction it is desirable that p and/or f canbe different for different printheads.

FIG. 9 shows an example of a printer controller 900 configured toimplement the above described methods.

In FIG. 9 a data processing system comprises a digital signal processor906 coupled to working memory 908 and having a print data input 902 toreceive data defining a pattern to be printed, and a distortion datainput 904 to receive distortion data, for example, determined asdescribed above either manually or, preferably, automatically by meansof an image capture and processing system. A data output 910 is providedto a head driver system 912 which provides output waveforms, forexample, of the type shown in FIG. 8, to one or more multiple nozzledroplet deposition type print heads. The data processing system is alsocoupled to permanent program memory 914 such as flashram or rom. Memory914 stores processor control codes for controlling DSP 906 to implementthe above described distortion compensation methods. In particularmemory 914 stores distortion compensation module code 914 a comprisinginitial head angle/translation determination code, headangle/translation adjustment code (for angle/translation adjustmentduring printing), and lateral pitch distortion compensation code;input/output code 914 b; and (optionally) distortion patterndetermination code 914 c for determining and/or modifying distortionpattern data. For example, this latter code may operate on the printdata to derive modified print data (and modified distortion data)requiring reduced distortion compensation, as outlined above. Optionallysome or all of the code stored in permanent program memory 914 may beprovided on a removable storage medium illustratively shown by floppydisk 916.

The invention is applicable to a range of printing applications ondistorted substrates. Examples include, but are not limited to:

-   -   Inkjet printing of an array of metallic lines on a distorted        substrate using a nanoparticle metal or a precursor metal ink,        or using a printable catalyst in combination with an electroless        plating process.    -   Inkjet printing of conducting, semiconducting, dielectric        polymer patterns for fabrication of an active or passive matrix        display array, such as a TFT array for an active matrix        electrophoretic or liquid crystal display, or an array of        light-emitting diodes for an emissive display or an array of        photodiodes for an image sensor device.    -   Graphic arts printing applications which require accurate        registration of different materials on a distorted substrate.

Embodiments of the invention are also applicable to other direct-writepatterning techniques which are not based on direct-write materialsdeposition. The technique is applicable in an entirely analogous way,for example, to direct-write laser patterning using a laser patterninghead comprising at least two focussed laser beams. Such focussed beamsmay be generated, for example, by passing laser light to a suitablephotomask which forms a light pattern when projected onto the substrate,or alternatively by using a digital mirror device or other light valvetechnology. Each beam may be turned on and off digitally by applying acontrol signal. The laser patterning head is moved relative to thesubstrate along a patterning direction. In this way a light exposurepattern on the substrate is generated that is analogous to the patternof material defined by the materials deposition unit above, andembodiments of techniques according to the present inventioncompensating for the distortion of an underlying pattern can be appliedanalogously. Each laser beam modifies the properties of at least onelayer of the substrate upon exposure, for example, by removing materialfrom the substrate by laser ablation, or by inducing a photochemicalreaction of said layer. This change in the properties of said layer isused to define a pattern on the substrate. In a preferred embodimentsaid pattern is defined by direct-write laser ablation.

In this case a patterning controller comprises at least one direct-writepatterning head with a plurality of patterning channels for patterningof a layer on a substrate, wherein said substrate (or said head) ismoveable in a patterning direction, said patterning head beingpositionable relative to said substrate by at least rotation about anaxis substantially perpendicular to said substrate at a patterningposition of said head; the controller comprising: a distortion datainput to receive substrate distortion information characterisingdistortion of said substrate to be patterned; a distortion compensationmodule to determine, responsive to said distortion information,patterning head adjustment data for controlling said patterning headrotation, preferably during patterning, to compensate for said substratedistortion; and an output to output data for controlling said patterningapparatus responsive to said patterning head adjustment data.

The technique is similarly applicable to non-periodic patterns such asthe circuit patterns in a logic circuit on a flexible substrate or on aprinted circuit board. The same technique of adjusting the rake angleand lateral position of each head can be used in this case to adjust forthe local distortion of a previously defined pattern in the y-directionperpendicular to the print direction.

The specific algorithms described above for adjusting the values ofα_(n) and s_(n) during the print swath provide a specific embodiment ofthe invention. However, a person skilled in the art that using theprinthead assembly described here other algorithms for correcting thesubstrate distortion can be used.

The present invention is not limited to the foregoing examples. Aspectsof the present invention include all novel and/or inventive aspects ofthe concepts described herein and all novel and/or inventivecombinations of the features described herein. The invention might alsobe used in combination with conventional distortion compensationtechniques, such as the technique described above of using a largenumber of heads to effectively create a grid much smaller than thenozzle pitch.

The applicant draws attention to the fact that the present inventionsmay include any feature or combination of features disclosed hereineither implicitly or explicitly or any generalisation thereof, withoutlimitation to the scope of any definitions set out above. In view of theforegoing description it will be evident to a person skilled in the artthat various modifications may be made within the scope of theinventions.

1. A controller for controlling direct-write apparatus, the apparatuscomprising at least one direct-write head with a plurality ofdirect-write channels for writing onto a substrate, at least one of saiddirect-write head and said substrate being moveable relative to theother in a writing direction, said head being further positionablerelative to said substrate by at least rotation about an axissubstantially perpendicular to said substrate at a writing position ofsaid head; the controller comprising: a distortion data input to receivesubstrate distortion information characterising distortion of saidsubstrate to be written; a distortion compensation module to determine,responsive to said distortion information, head adjustment data forcontrolling said head rotation to compensate for said substratedistortion; and an output to output data for controlling said printingapparatus responsive to said head adjustment data.
 2. A controller asclaimed in claim 1 wherein said substrate distortion information definesa pattern of distortion of said substrate in two dimensions, whereinsaid apparatus is configured for control of both said head rotation andhead translation in a lateral direction substantially perpendicular tosaid writing direction, and wherein said distortion compensation moduleis configured to determine head adjustment data comprising head rotationdata and head translation data for adjusting both said head rotation andsaid head translation responsive to said distortion information.
 3. Acontroller as claimed in claim 2 further comprising a patterning datainput to receive data defining a direct-write pattern; and wherein saidoutput data comprises data for controlling said direct-write headresponsive to both said head adjustment data and said direct-writepattern data, for controlling said head rotation and said headtranslation during motion of said direct-write head in said writingdirection to compensate for pitch distortion in at least a lateraldirection substantially perpendicular to said writing direction duringwriting of said direct-write pattern by said apparatus.
 4. A controlleras claimed in claim 3 wherein said controller is configured to outputdata for controlling said direct-write head rotation responsive todistortion information characterising pitch of a periodic pattern at aposition of said direct-write head on said substrate with respect towhich the direct-write pattern is to be aligned.
 5. A controller asclaimed in claim 4 wherein said controller is configured to output datafor controlling said direct-write head translation to compensate forcontrol of said direct-write head rotation.
 6. A controller as claimedin claim 1 wherein said direct-write apparatus comprises dropletdeposition-type printing apparatus, and wherein said direct-write headwith a plurality of channels comprises a droplet deposition-type printhead with a plurality of nozzles, and wherein said writing comprisesprinting.
 7. A controller as claimed in claim 6 wherein said output dataincludes timing data for controlling timing of said droplet depositionto compensate for pitch distortion of said substrate in said printdirection.
 8. A controller as claimed in claim 1 for controllingdirect-write apparatus having a plurality of said heads, and whereinsaid output data comprises data for controlling said heads to movetogether to compensate for said substrate distortion.
 9. A controller asclaimed in claim 1 further comprising: data memory for storing saiddistortion information; instruction memory storing processorimplementable instructions; and a processor coupled to said data memory,to said instruction memory, to said distortion data input, and to saidoutput, and operable to load and process said distortion information inaccordance with said instructions; and wherein said instructionscomprise instructions to implement said distortion compensation moduleto determine said head adjustment data, and instructions to determineand output said output data.
 10. A data carrier carrying theinstructions of claim
 9. 11. Printing apparatus comprising at least onedroplet deposition-type print head with a plurality of nozzles fordroplet deposition onto a substrate, at least one of said print head andsaid substrate being moveable relative to the other in a printdirection, said print head being further positionable relative to saidsubstrate by at least rotation about an axis substantially perpendicularto said substrate at a printing position of said head and translation ina lateral direction substantially perpendicular to said printingdirection; and wherein the apparatus further includes a controller asclaimed in claim
 1. 12. A method of compensating for pitch distortion ofa substrate onto which material is to be deposited by a printer havingat least one print head with a plurality of material depositionchannels, said pitch distortion comprising distortion of the pitch of apattern with respect to which said deposited material is to be aligned,the method comprising: adjusting a rake angle of said print head duringprinting to compensate for said pitch distortion, said rake anglecomprising an angle of said print head to a direction of said printing.13. A method as claimed in claim 12 further comprising adjusting alateral position of said print head in combination with said rake angleadjusting during said printing to compensate for said pitch distortion,said lateral position comprising a position of said print head along adirection substantially perpendicular to said printing direction.
 14. Amethod as claimed in claim 12 further comprising inputting distortiondata for said substrate, said distortion data providing information onsaid pitch distortion over a two-dimensional region of said substrate,and wherein said adjusting is performed responsive to said pitchdistortion information.
 15. A method as claimed in claim 14 furthercomprising processing said pitch distortion information prior to saidadjusting to reduce a degree of said adjusting.
 16. A method as claimedin claim 14 wherein said rake adjusting is responsive to a pitchdistortion at a position of said print head.
 17. A method as claimed inclaim 12 wherein said printer has a plurality of said print heads, andwherein said adjusting comprises adjusting rake angles of said pluralityof print heads, the method further comprising adjusting one or morespacings between said print heads in combination with said adjusting ofrake angles to compensate for said pitch distortion.
 18. A method asclaimed in claim 12 further comprising altering a timing of saidprinting to compensate for pitch distortion along said direction ofprinting.
 19. A method as claimed in claim 18 wherein said printing withsaid material deposition channels is performed in accordance with aprint drive waveform for each channel, and wherein the method furthercomprises altering one or both of a frequency and a phase of each saiddrive waveform during printing.
 20. A method of compensating for pitchdistortion of a substrate which is to be written by a direct-writeapparatus having at least one direct write head with a plurality ofdirect-write channels, said pitch distortion comprising distortion ofthe pitch of a pattern with respect to which said writing is to bealigned, writing with said direct-write channels being performed inaccordance with a write drive waveform for each channel, the methodcomprising: moving at least one of said substrate and said head relativeto the other in a writing direction during writing; and altering one orboth of a frequency and a phase of each said drive waveform during saidwriting in response to distortion data providing information on saidpitch distortion over a two-dimensional region of said substrate tocompensate for pitch distortion in said writing direction.
 21. A carriercarrying computer program code to, when running, implement the method ofclaim
 12. 22. A printer controller including the carrier of claim 21.23. A method of fabricating an electronic device on a flexiblesubstrate, the method including defining at least one layer of thedevice using a technique which compensates for local distortion of thesubstrate by print head rotation.
 24. A carrier carrying computerprogram code to, when running, implement the method of claim
 20. 25. Aprinter controller including the carrier of claim 24.